Polymyxins were discovered in 1947 as antibiotics produced by Bacillus polymyxa. Polymyxins are antibiotic decapeptides containing a heptapeptide ring and a N-terminal amide coupled fatty acid. Today, two commercial Polymyxin mixtures are in clinical use; Polymyxin B and Polymyxin E (Colistin). Both mixtures comprise a variety of components as described in J Chromatogr A. 2002 Nov. 8; 976(1-2):65-78 by Goevaerts et al and in Talanta 2011 Feb. 15; 83(5):1521-9 by Van den Bossche et al. According to the European pharmacopoeia, Colistin should comprise more than 77% of Polymyxin E1, E2, E3, E1-i and E1-7MOA, but less than 10% of each of the minor components Polymyxin E3, E1-i and E1-7MOA.
Due to toxicity associated with Colistin, the mixture was improved by sulfomethylation in the 1950'ties. The sulfomethylated Colistin is called Colistimethate sodium (CMS) which has been considered to be a prodrug of Colistin. CMS is still in clinical use as a last-line treatment option for multidrug-resistant organisms such as Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumonia and other Gram negative pathogens. For many years, solutions of CMS have also been administered by nebulization into the lungs of patients with cystic fibrosis (CF) to manage colonization or infections caused by P. aeruginosa. 
Today, the increasing problem with infections caused by antibiotic resistant pathogens has resulted in an increased use of products such as Colistin.
The fact that commercial CMS products contain a complex mixture of derivatives of different Polymyxins has several unfortunate consequences. The foremost of these relates to the therapeutic value of any marketed product. Since CMS may be considered a antibiotic-reservoir once injected or inhaled into the body, it is of importance that therapeutic levels of the activated compounds are reached before renal or metabolic clearance. Thus, a sulfomethylated polymyxin containing two sulfomethyl substituents only, provides a well-defined product with respect to molecular weight and charge.
An object of the present invention is therefore to provide improved polymyxin compounds and compositions useful as antimicrobial agents, and to overcome or at least mitigate the disadvantages of the prior art products.